A synchronous motor has a stator and a rotor supported in the inner periphery of the stator and is capable of being locally exciting, being structurally the same as the stator of a common induction motor. Generally, the synchronous reluctance motor is well known as a motor which is simply structured so as to not need electric current channels and permanent magnets in the rotor.
Presently, synchronous reluctance motors having a good power factor and efficiency by a structurally improved rotor are well known. In a Japanese Patent Application Laid-Open Publication No. 8(1996)-331783 published on Dec. 13, 1966, a permanent-magnetic motor additionally generating a reluctance torque by using a synchronous reluctance motor to generate a rotational torque is proposed. In other words, the above permanent-magnetic motor can be regarded as the synchronous reluctance motor generating the rotational torque by mainly permanent magnets. This synchronous reluctance motor (the disclosed permanent-magnetic motor) has a rotor having some pairs of slots that are formed approximately parallel to each other in the circumferential direction of the rotor. The slots are extended adjacent to the outer peripheral surface of the rotor, and the permanent magnets are secured in the pairs of slots.
The synchronous reluctance motor is designed with the aim of generating a larger torque and higher power by using reluctance torque. The reluctance torque is generated by a difference between the inductance Ld of the rotor in a d-axis direction (defined by connecting a center-point of the permanent magnet in circumferential direction of the rotor with a rotational center of the rotor), and an induction Lq of the rotor in a q-axis direction (defined as a direction rotated relative to the d-axis direction by 90 electrical degrees).
As described above, this synchronous reluctance motor is aimed to use the reluctance torque. But the synchronous reluctance motor is constructed to mainly use a magnet torque generated between the permanent magnet and the rotor, and not to positively and sufficiently use the reluctance torque.
In the rotor of this synchronous reluctance motor, portions of the rotor defined between the permanent magnets neighboring each other in the circumferential direction (or distance S defined between the permanent magnets neighboring each other in the circumferential direction) are determined to be as small or narrow as possible. To the contrary, the permanent magnets are designed to be as large in size as possible so that the magnetic flux will not leak outside of the distance S, and for efficiently using the magnet torque.
In this synchronous reluctance motor, the inductance Lq should be large, while, the inductance Ld should be small for generating the reluctance torque. Only for determining the inductances Ld and Lq as above, the distance S should be determined to be large or wide. Because the inductance Lq is increased due to the increased gap S, the inductance Ld is not so increased that the magnetic circuit connecting magnetic pole portions of the stator is formed in the rotor. If the reluctance torque is increased relative to the total torque generated by the synchronous reluctance motor, a torque ripple may occur. To reduce the torque ripple, a plurality of permanent magnets need to be disposed in the radial direction of the rotor, but then, for example, the manufacturing cost of the rotor will be increased.
Thus the synchronous reluctance motor capable of generating the large total torque and reducing the torque ripple, and further manufactured at low cost is desired.